Slept on my back after Brazilian Butt lift. (photo)

It does not appear as if there was any damage done to your surgery. It is always recommended that you consult with your surgeon as well with any concerns or questions you may have.

Thanks for your question.
There is no scientific evidence that sitting on a BBL will affect the results. However, it makes intuitive sense not to traumatize the areas where fat was placed and is currently healing. I suggest you check with your surgeon for his or her recommendation. 
Take care,
Moises Salama M.D., Board Certified Plastic Surgeon

I think it is best not to sleep on your back after a BBL but it is certainly not as bad as sitting. 
I recommend my patients buy a horseshoe type or pregnancy pillow—also referred to as a husband pillow that wraps around you and can prevent you from rolling over onto your back at night but the reality is that everyone likely spends sometime sleeping on their back at night inadvertently and they do fine so I would not be too concerned.